Which came first, the salt or the saltcedar?


A quantitative study of soil and groundwater chemistry along the Middle Rio Grande, New Mexico

by Michelle Cederborg, Ph.D. Candidate, University of Denver, Colorado

Figure 1. Saltcedar along the Middle Rio Grande, New Mexico in June of 2006 featuring Dr. Ken Lair of the Bureau of Reclamation, Denver Office.

Saltcedar (Tamarix spp, a.k.a. tamarisk; Figure 1) is an invasive, non-native plant that has taken a permanent foothold in southwestern United States riparian ecosystems. Whole ecosystem properties have been altered by saltcedar invasion (Vitousek 1990). One of the most critical, yet least understood, is increased soil and surface water salinity.

As the common name “saltcedar” implies, one of the most often cited mechanisms of ecosystem change by this species stems from its ability to sequester salts in its tissues (Thomson et al. 1969). This deciduous tree can extract salts from the groundwater, excrete these compounds through leaf tissue, and deposit them onto the soil surface through mature leaf senescence and exudation (Ladenburger et al. 2006, Lesica and DeLuca 2004, Shafroth et al. 1995). This alteration of surface salt concentrations has the ability to disrupt soil nutrient dynamics and contaminate surface waters with potential negative consequences for local plant and animal populations. Knowledge of invasion mechanisms and long-term environmental impacts of saltcedar is fundamental for understanding habitat restoration and revegetation potential upon its removal.

Excess soluble salts, typically dominated by sodium compounds, begin to become troublesome for plant species when they accumulate in soils. Under high concentrations of sodium, elements such as calcium and magnesium are often lost from the system leaving soils deficient in these plant-essential nutrients. In addition, the forces that bind clay particles together are disrupted when too many large sodium ions are present. When this separation occurs, the clay particles expand, causing swelling and soil dispersion. Soil dispersion leads to reduced water infiltration which can result in reduced plant-available water and increased runoff and soil erosion.

The outcome becomes even more dismal for plants without a mechanism for compartmentalizing salts within their tissues. Unlike saltcedar, most native riparian species do not possess salt glands (structures that serve to sequester and dispose of ingested salts without harm to the plant’s internal structure and function). Native riparian plants must therefore regulate their salt uptake at the root-soil interface which can lead to a build-up of excess salts outside of plant roots. As salt concentrations rise above that of the plant’s internal environment, the osmotic potential of the soil decreases and plants become less able to extract water from the soil. Even with access to an abundant water source the plant suffers from drought stress, a condition that becomes more severe with increased salinity. Elevated salt concentrations are well documented to negatively impact establishment and growth of native riparian species such as cottonwood and willow (Glenn et al. 1998, Rowland et al. 2004, Vandersande et al. 2001) and salt-cedar has even been labeled allelopathic via this mechanism of salt retranslocation (Brock 1994, Brotherson and Field 1987). No quantitative support of this assumption in the context of environmental factors that may also influence surface salt levels, however, currently exists in the published literature.

Environmental factors such as geology, climate, and anthropogenic practices could predispose an area to elevated salt levels. Areas where groundwater comes in contact with volcanic vents that transport magnesium and sodium ions from the earth’s mantle and sites where the rock types contain high concentrations of rapidly weathered salt constituents tend to have higher levels of environmental salinity. In addition, soil salinity is typically exacerbated in arid regions due to a lack of sufficient rainfall that would leach and transport salt deposits, and high evaporation rates; such factors tend to further concentrate salts in the surface soils. The most often cited cause of elevated salt concentrations along streams and rivers is river regulation (Busch and Smith 1995, Stromberg 2001). Flow-regulated and channelized river stretches can develop saline bankside conditions because they are no longer subject to periodic overbank flooding which washes salts from the soil.

Since saltcedar is a facultative halophyte and can therefore tolerate and even thrive in conditions of elevated salinity (Glenn et al. 1998), it is questionable whether invasion of this opportunist is the cause or the effect of solute imbalances. In order to accurately charge saltcedar with environmental salt-loading, we need to take into account other potential sources contributing to elevated salt concentrations. Specifically, we need to know which came first, the salt or the saltcedar.

Figure 2. Preliminary results for research on the impact of saltcedar on environmental salinity. Soil salinity (electrical conductivity – EC, measured in mmhos/cm) sampled a) inside and outside the levees, and b) in high and low density saltcedar stands. Soil samples were collected along the Middle Rio Grande, New Mexico in July of 2006.

If saltcedar does, in fact, directly contribute to soil and surface water salinity, then we might expect that the magnitude of its impact would be influenced by stand characteristics such as average individual age and density and whether there is sufficient flooding to flush the soil and thereby lower salinity. In addition, a thorough analysis of the relationship between saltcedar and surface salt levels associated with an environment prone to high salt concentrations (i.e. one that contains salt-rich rock types, arid to semi-arid conditions, a groundwater junction with volcanic vents, etc.) could potentially provide information about the worst possible scenario of surface salt escalation in invaded areas.

We chose to examine the relationship between saltcedar presence and elevated surface salinity within saltcedar-infested and native-dominated sites along a system that receives salt inputs from all of the fore mentioned sources, the Middle Rio Grande in central New Mexico. We selected saltcedar stands across a gradient of aboveground saltcedar ages (five to 40 years of age) and densities (native-dominated to complete saltcedar cover) to determine how these stand attributes relate to surface salt-loading. In addition, we chose stands in the proximal, active floodplain (exposed to annual flood events) and in the more distal, upper floodplain terraces (typically outside of the levees and deprived of overbank flooding) to hopefully isolate potential saltcedar-induced alterations to soils and surface water quality as well as address hydrologic impacts on environmental salinity.

Preliminary data collected in 2006 show a strong relationship between surface soil salinity and flooding (Figure 2a). Areas sampled outside the active floodplain have significantly higher salt levels when compared with areas exposed to periodic flooding. These results would suggest that flooding is a dominant factor responsible for elevated salt concentrations along this river system.

Our preliminary findings also show that areas with greater saltcedar densities have higher soil salt levels than areas with reduced saltcedar cover (Figure 2b). Similarly, we observed that areas dominated by saltcedar within the active floodplain contained higher levels of surface soil salts when compared with adjacent native-dominated sites. These results support the claim that saltcedar is actively contributing salts to the soil surface even in the presence of flooding.

Preliminary results also suggest that beyond a certain age of saltcedar, surface soil salinity begins to decrease (Figure 3). This finding is contrary to what would be expected in an environment that has been subjected to repeated salt inputs for longer periods. Decadent saltcedar growth, however, tends to consist primarily of older woody material and has lower leaf area indices than younger saltcedar stands. The reduced saltcedar leaf area in older infestations likely leads to less salt redistribution to the soil surface through leaf material. In addition, the dense aboveground woody network associated with older growth may reduce solar radiation and elevate localized humidity levels at the soil surface; both of which can contribute to reduced capillary rise of salts to surface soils in arid regions.

The extent to which saltcedar invasion alters soil chemistry has often been assumed to be large, but this claim is largely unverified. We collected over one thousand surface soils and groundwater samples in 2007 to better assess the role of saltcedar versus other factors (i.e. hydrology) in determining floodplain salinity. Soils and groundwater analyses will be directly compared to detailed vegetation surveys to determine how surface soil and groundwater salinity relate to species composition, density, and in the case of saltcedar, average individual age. At this time, it is assumed that decadent saltcedar stands may have already altered the soil to such a degree that any attempt at restoration would be futile. A complete analysis of this larger dataset will allow us to determine if this is indeed the case, or if, as preliminary data suggest, these older stands may actually have the least saline soils and are therefore more conducive to native riparian species revegetation.

Understanding the relationship between saltcedar stand characteristics and associated edaphic and surface water impacts is critical for the development of site-specific management strategies for saltcedar-invaded areas. If a relationship is found between saltcedar stand characteristics and surface soil and groundwater salinity, then land managers can use this knowledge to prioritize restoration efforts, which should focus on those infestations most feasibly revegetated. A diagnosis of elevated salinity could also be used to assign a more appropriate revegetation species mixture (i.e. one that can withstand elevated salt concentrations until the site becomes more favorable for native riparian species growth and survival). Since salt-cedar has been shown to be a poor competitor with native species at the seedling stage (Sher et al. 2002), it is imperative that a successful cover crop become established in order to prevent the re-invasion of this pervasive non-native. The complicated and expensive removal techniques necessary to control saltcedar heightens the need for educated land management to ensure funds are allocated to areas and restoration project designs with the greatest potential for native species revegetation success.

Literature Cited:
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